Patellar prosthesis and method of making the same

ABSTRACT

A composite metal/plastic patellar prosthesis for implantation in the human body to replace the articulating surface of the patella is provided. The prosthesis includes a polymeric articulating surface portion and a metal backing having an anterior surface adapted to be attached to the posterior surface of a resected patella, and a dome or conically shaped posterior surface to which the polymeric articulating surface portion is heat and pressure molded. The metal backing portion has at least one aperture extending therethrough which has a greater cross-sectional area on the anterior side than on the posterior side into which an anchoring peg integral with the antrior surface of the polymeric articulating surface portion is molded. The metal and plastic components of the composite are coextensive in size and shape.

BACKGROUND OF THE INVENTION

The present invention relates to an improved patellar prosthesis andmethod of making the same.

Various prosthetic devices for replacement of the articulating surfaceof the patella or knee cap have been designed for implantation in caseswhere replacement of the natural articulating surface of the patella isindicated. Generally, the posterior surface of the patella is resectedand replaced by an artificial articulating surface.

Various patellar prostheses are known in the art and availablecommercially to surgeons. Examples of such are the designs shown in U.S.Pat. No. 4,240,162 issued to Devas on Dec. 23, 1980 in which a two-partsnapped-together design is disclosed. Commercially available implantsare shown, for example, in Publication No. 322-688T of Dow CorningWright Corporation, 5677 Airline Road. Arlington. TN 38002 entitled"Whiteside Ortholoc II Patella Implants". Similar designs having a metalbacking within a polymeric surface portion are shown, for example, in apublication by Howmedica, Inc., Orthopedics Division, 359 VeteransBoulevard, Rutherford, NJ 07070 identified as No. H2050, dated 3/83,entitled "The PCA Revision Total Knee System" at page 4. Such prostheseshave been known to fail by virtue of separation of the polymericarticulating surface from the metal backing.

In light of these various designs, a need has existed for an improvedpatellar prosthesis having resistance to shear forces experienced duringarticulation of the patellar implant against an articulating surface ofa femoral prosthesis. The present invention provides such increasedresistance by a design in which shear forces imposed against the side ofthe patellar implant are, in part, converted to compression forces. Theinvention further provides a means and method of molding the polymericarticulating surface onto the metal backing in such fashion as toimprove the mechanical locking, and thus resistance to wear, between thecomponents of the prosthesis.

Briefly summarized, the present invention provides a composite patellarprosthesis for implantation in the human body to replace thearticulating surface of the patella which includes a polymericarticulating surface portion, and a dome shaped metal backing which hasan anterior surface adapted to be attached to the posterior surface of aresected patella, preferably by means of integral attachment pegs. Thedome shaped, or otherwise centrally elevated posterior surface isadapted to engage the polymeric articulating surface portion which ismolded under heat and pressure to conform to it. The metal backingportion is provided with at least one, and preferably several aperturesextending therethrough which have a greater cross-sectional area on theanterior side than on the posterior side. These apertures are eachadapted to fixedly receive an anchoring peg integral with the anteriorsurface of the polymeric articulating surface portion, the pegs beingformed under heat and pressure to precisely fill the apertures. It isalso preferred that the anterior surface of the polymeric articulatingsurface portion is coextensive in size and shape with the metal backingportion is molded to conform to the contour thereof. The articulatingsurface portion has a generally convex posterior articulating surface.The apertures through the metal backing are greater in area on theanterior side than on the posterior side so that the pegs firmly lockthe two parts of the composite together. The anterior side may, forexample, be counterbored or the apertures may be tapered or flared so asto be of greater size on the anterior side.

DRAWINGS

The invention will be further explained with reference to theaccompanying drawings wherein:

FIG. 1 is a perspective view of a patellar prosthesis of the presentinvention, and,

FIG. 2 is a cross-sectional view of such a prosthesis.

DETAILED DESCRIPTION

Referring more specifically to the drawings, patellar prosthesis 10 isformed from a rigid, preferably metallic backing portion 12, provided onits anterior surface with integrally formed securing pegs 13 which areprovided to secure the prosthesis to the posterior side of a resectedpatella. In accordance with conventional practice, the anterior surfaceof metallic attachment portion 12 can be provided with a porous layersuch as sintered metal microbeads 14 to allow for tissue ingrowth.

Secured to the posterior side of metallic supporting layer 12 is apolymeric surface layer 15 preferably formed from a thermoplasticmaterial such as ultra high molecular weight polyethylene. Otherbiologically acceptable moldable materials such as polypropylene or thelike can be substituted in articulating surface 15. Polymericarticulating surface portion 15 is preferably formed so that it is thesame size and shape as the metal backing element 12, and the anteriorside of the articulating surface member is pressure molded so that itflows and conforms to the posterior surface of metal backing element 12.

The polymeric surface portion is affixed to the metal backing portion bymeans of one or more integral pegs 16 which are molded by heat andpressure into an apertures extending through metal backing member 12.The apertures are configured so that the anterior side is larger thanthe posterior side, for example by means of a counter-bored portion 17on the anterior side, or by means of slightly tapered or anteriorlyflared openings. The preferred embodiment, shown in the drawings,utilizes one central peg 16 along with four smaller pegs 18 all of whichare heat and pressure molded into the apertures to provide a secureattachment of the backing member to the wear surface.

Also as seen best in FIG. 2, the central portion of the metal backingportion 12 is centrally thickened into a domed shape. This configurationhas been found to be superior to prior art devices in which flatabutting surfaces were employed. Such flat surfaces may be subjected toshearing forces as the prosthesis wears, whereas the slightly domeshaped configuration of the present prosthesis causes a vector of thoseshearing forces to be converted into compression forces which have beendiscovered to be more resistant to failure at the junction between theparts of the composite prosthesis.

The prosthesis of the present invention is formed by providing a plasticblank of the articulating surface which is provided with a central peghaving a height slightly greater than the thickness of the metal backingmember 12. When molded under heat and pressure the anterior surface ofthe plastic member is forced to assume the contour of the metal backingmember and the central, peg flows into the undercut portion 17 of thecentral aperture. The apertures spaced around the central aperture,however, are of a smaller diameter and it has been found that theplastic material of the articulating surface 15 will flow into suchapertures under heat and pressure, to fill the apertures as shown inFIG. 2. This method of construction obviates the need for formation ofadditional pegs on the anterior surface of the articulating portionblank. If desired, however, additional pegs could be employed,particularly if it is desired to use somewhat larger diameter aperturesthan indicated in FIG. 1.

The prosthesis of the present invention is implanted in accordance withusual surgical procedures followed for implantation of patellarprostheses. Such procedures, therefore, will not be elaborated on in thepresent disclosure.

While the foregoing specification give a detailed description ofspecific embodiments of the invention for the purpose of illustration,it will be understood that many of the details given may be variedconsiderably by those skilled in the art without departing from thespirit and the scope of the invention.

That what is claimed is:
 1. A patellar prosthesis for implantation inthe human body to replace the articulating surface of the patellacomprising:a polymeric articulating surface portion, and, a metalbacking having an anterior surface adapted to be attached to theposterior surface of a resected patella, and a posterior surface adaptedto engage the polymeric articulating surface portion, said metal backingportion having a central portion that is thicker than the outer edgesthereof, said central portion being elevated on the posterior surface ofsaid backing and having at least one aperture extending therethroughwhich has a greater cross-sectional area on the anterior side than onthe posterior side thereof, adapted to fixedly receive an anchoring pegintegral with the anterior surface of the polymeric articulating surfaceportion, said anterior surface of said articulating surface portionbeing coextensive in size and shape with said metal backing portion andbeing molded to conform to the contour thereof, said articulatingsurface portion have a generally convex posterior articulating surface,said anchoring peg being molded into said aperture and fillingsubstantially the entire volume thereof.
 2. A prosthesis according toclaim 1 wherein said metal backing also contains a plurality of smallapertures each of which is of increasing diameter in the posterior toanterior direction.
 3. A method of making a patellar prosthesis forimplantation in the human body to replace the articulating surface ofthe patella comprising:providing a polymeric articulating surfaceportion blank, and, a metal backing portion which has an anteriorsurface adapted to be attached to the posterior surface of a resectedpatella, and a dome shaped posterior surface adapted to engage thepolymeric articulating surface portion, said metal backing portionhaving at least one aperture extending therethrough which has a greatercross-sectional area on the anterior side than on the posterior sidethereof, adapted to fixedly receive an anchoring peg integral with theanterior surface of the polymeric articulating surface portion, saidanterior surface of said articulating surface portion blank beingcoextensive in size and shape with said metal backing portion andcapable of being molded to conform to the contour thereof, saidarticulating surface portion having a generally convex posteriorarticulating surface. molding said polymeric surface portion blank ontothe posterior side of said metal backing under heat and pressure so thatsaid anchoring peg is molded into said aperture and fills substantiallythe entire volume thereof.
 4. A method according to claim 3 wherein saidmetal backing contains small diameter apertures therethrough which areof increasing diameter in the posterior to anterior direction, and saidapertures become filled with said polymeric material under applicationof such heat and pressure.